This invention relates to a control system for the internal combustion engine or more in particular to an electronically controlled fuel supply system for controlling the amount of fuel supply by measuring the flow rate of air sucked into the engine.
Generally, in an electronically controlled fuel supply system, fuel is supplied into the air path leading to the engine by a fuel injector, which includes an injection valve opened in synchronism with the engine rotation and kept open for a predetermined period of time. This valve-open period is the fuel injection time regulated by the electronically controlled fuel supply system in such a manner as to attain a predetermined air-fuel ratio for the amount of air sucked in. Theoretically, the air-fuel ratio is controlled to obtain the theoretical air-fuel ratio at which oxygen held in the sucked air and injected fuel is used for combustion in proper quantities. The precision of air-fuel ratio depends on the detection accuracy of a sucked air flow rate sensor and the response accuracy of the fuel injection valve. These accuracies of detection and response change with time after a protracted use of the engine. Therefore, accurate control is impossible merely by controlling the fuel injection time against the sucked air flow rate in such a manner as to attain a certain fixed air-fuel ratio.
In order to eliminate the above-mentioned disadvantage, a system has been commercialized by which the oxygen concentration in the exhaust gas from the engine is detected and the fuel injection time against the sucked air flow rate is controlled in such a manner that the detected oxygen concentration corresponds to the theoretical air-fuel ratio. This system takes advantage of the fact that the oxygen concentration of the exhaust gas is sharply reduced with the increase in the air-fuel ratio in the neighbourhood of the theoretical air-fuel ratio. The oxygen concentration in the exhaust gas is generally detected by an oxygen sensor including an air-permeable zirconia solid electrolyte. This control system generally employs a closed loop and therefore is effective in control to hold the engine operating condition at the theoretical air-fuel ratio. When the engine is required to be operated in the condition displaced from the theoretical air-fuel ratio as in the automobile warm-up, acceleration, running up or down a comparatively steep slope or along a freeway, however, such a control system cannot maintain the proper operating condition, since it is necessary to operate the engine with the mixture gas whose the fuel concentration is thicker or thinner than the theoretical air-fuel ratio. Further, when the air flow rate is sharply changed, such a control system cannot maintain the proper operating condition, since the time lag in which the air-fuel ratio information is transferred from oxygen sensor to injection valve is too long. In such special operating conditions, the closed loop control based on the data of the theoretical air-fuel ratio from the oxygen sensor is impossible, so that an open loop control is employed in which control is based on a predetermined air-fuel ratio determined as suitable for each of the above-mentioned special operating conditions. In such an open loop control, it is quite impossible to correct the change with time of the functions or response of the air flow rate sensor or injector. In such special operating conditions, therefore, neither fuel consumption is saved nor exhaust gas is purified properly and smoothly.